CHAPTER 1 



The Integument 



The key to much of the physiology of insects is to be found in the 

 nature of their cuticle. As was first shown by Haeckel, the cuticle is 

 the product of a single layer of epidermal cells. It is often described 

 as being composed of non-living material; but in fact the epidermal 

 cells give off fine protoplasmic filaments, the so-called 'pore-canals', 

 which run through the substance of the cuticle and often come with- 

 in less than a micron of the surface. 



As described from stained sections the cuticle consists of two 

 primary layers, the endocuticle which makes up the greater part, and 

 a thin refractile epicuticle on the surface, usually not more than one 

 micron in thickness. In the harder regions of the integument the 

 outer half or third of the endocuticle is converted to a deep brown or 

 amber layer, termed the exocuticle. In stained sections the epicuticle 

 and exocuticle appear homogeneous and structureless; the endo- 

 cuticle shows more or less conspicuous horizontal lamellae. The 

 pore canals can rarely be seen. But if the cuticle of certain insects, 

 such as Periplaneta, Tenebrio, Rhoclnius, &c, is cut in the fresh state 

 with the freezing microtome and mounted in water without staining, 

 the pore-canals are conspicuous objects (often far more conspicuous 

 than the horizontal lamellae) running right through the endo- and 

 exocuticle. Sometimes their contents may be hardened in the exo- 

 cuticle of the mature insect; but very often, if the fresh sections are 

 dried in air before being mounted, their moist contents contract and 

 the canals become filled with fine threads of gas. Clearly the cuticle is 

 far from being dead : it often contains threads of living cytoplasm 

 coming almost to the surface; in this respect it resembles the bone 

 and dentine of vertebrates. 



The 'living' character of the cuticle is evidenced by the fact that 

 under the action of some humoral factor conveyed by the nerves it 



